The long term objective of this research is to elucidate the detailed molecular mechanisms of proteins that mediate cell-cell fusion events. cDNAs encoding the mature subunits of PH-30, a protein involved in sperm- egg fusion, have recently been cloned. The deduced sequences suggest that PH-30 resembles viral fusion proteins both in its membrane topology and predicted functions. The alpha-subunit contains a potential fusion peptide similar to those seen in viral fusion proteins. The beta-subunit contains an integrin ligand domain. Two hypotheses raised by these findings are: (i) PH-30-(beta) binds sperm to an integrin on the egg plasma membrane. (ii) PH-30-(alpha) induces fusion between sperm and egg plasma membranes. The major goal of this proposal is to test these hypotheses. The specific aims are to: 1. further characterize cDNAs encoding PH-30 alpha and beta; 2. test whether PH-30 is necessary and sufficient for binding to and fusion with the egg plasma membrane; 3. identify and clone the cDNA encoding the receptor for PH-30 on the egg plasma membrane; and 4. test whether the PH-30 receptor is necessary and sufficient for PH-30- mediated binding and fusion. The experimental design is modeled on previous studies of viral fusion proteins, viral receptors, and integrins. It will employ a combination of biochemical, immunological, and molecular biological approaches. In addition to its relevance to the basic cell biological processes of membrane fusion, cell-cell fusion, cell-cell adhesion, and adhesion- mediated signalling, the project has a high degree of health relatedness. PH-30 has a disintegrin domain. By analogy with the use of peptide analogs of platelet disintegrins as anticoagulants, peptide analogs of the PH-30 disintegrin domain should block fertilization. Hence the project should lead to the development of novel contraceptive agents and may shed light on the molecular bases of certain forms of male infertility.